Tài liệu High-Performance Parallel Database Processing and Grid Databases- P5 pptx

180 Chapter 7 Parallel Indexing the FRI-1 structure. Note that the global index is replicated to the three processors. In the diagram, the data pointers are not shown. However, one can imagine that each key in the leaf nodes has a data pointer going to the correct record, and each record will have three incoming data pointers. FRI-3 is quite similar to PRI-1, except that the table partitioning for FRI-3 is not the same as the indexed attribute. For example, the table partitioning is based on the Name field and uses a range partitioning, whereas the index is on the ID field. However, the similarity is that the index is fully replicated, and each of the records will also have n incoming data pointers, where n is the number of replication of the index. Figure 7.13 shows an example of the FRI-3. Once again, the data pointers are not shown in the diagram. It is clear from the two variations discussed above (i.e., FRI-1 and FRI-3) that variation 2 is not applicable for FRI structures, because the index is fully replicated. Unlike the other variations 2 (i.e., NRI-2 and PRI-2), they exist because the index is partitioned, and part of the global index on a particular processor is built upon the records located at that processor. If the index is fully replicated, there will not be any structure like this, because the index located at a processor cannot be built purely from the records located at that processor alone. This is why FRI-2 does not exist. 7.3 INDEX MAINTENANCE In this section, we examine the various issues and complexities related to main- taining different parallel index structures. Index maintenance covers insertion and deletion of index nodes. The general steps for index maintenance are as follows: ž Insert/delete a record to the table (carried out in processor p 1 ), ž Insert/delete an index node to/from the index tree (carried out in processor p 2 ), and ž Update the data pointers. In the last step above, if it is an insertion operation, a data pointer is created from the new index key to the new inserted record. If it is a deletion operation, a deletion of the data pointer takes place. Parallel index maintenance essentially concerns the following two issues: ž Whether p 1 D p 2 . This relates to the data pointer complexity. ž Whether maintaining an index (insert or delete) involves multiple processors. This issue relates to the restructuring of the index tree itself. The simplest form of index maintenance is where p 1 D p 2 and the inser- tion/deletion of an index node involves a single processor only. These two issues for each of the parallel indexing structures are discussed next. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Processor 1 23 Adams 37 Chris 46 Eric 92 Fred 48 Greg 78 Oprah 28 Tracey 39 Uma 8 Agnes Name: −x Processor 3 59 Johanna 74 Norman 16 Queenie 20 Ross 24 Susan 69 Yuliana 75 Zorro 49 Bonnie Name: −x x = vowel (i,o,u) x = consonant Processor 2 65 Bernard 60 David 71 Harold 56 Ian 18 Kathy 21 Larry 10 Mary 15 Peter 43 Vera Name: −x 47 Wenny 50 Xena 33 Caroline 38 Dennis x = vowel (a,e) o8 o10 o15 o28 o33 o37 o46 o47 o48 o38 o39 o43 o49 o 50 o56 o65 o69 o71 o16 o18 o23 o24 o20 o21 o59 o 60 o74 o75 o78 o92 15 43 56 37 18 21 24 71 75 48 60 8 Processor 1 o o10 o15 o 28 o33 o37 o46 o47 o48 o38 o39 o43 o49 o50 o56 o 65 o69 o71 o16 o18 o23 o24 o20 o21 o59 o60 o74 o75 o78 o92 15 43 56 37 18 21 24 71 75 48 60 Processor 2 o8 o10 o15 o28 o33 o37 o46 o47 o48 o38 o39 o43 o49 o50 o56 o65 o69 o71o16 o18 o23 o24 o20 o21 o59 o60 o74 o75 o78 o92 15 43 56 37 18 21 24 71 75 48 60 Processor 3 Figure 7.13 FRI-3 (index partitioning attribute 6D table partitioning attribute) 181 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 182 Chapter 7 Parallel Indexing 7.3.1 Maintaining a Parallel Nonreplicated Index Maintenance of the NRI structures basically involves a single processor. Hence, the subject is really whether p 1 is equal to p 2 . For the NRI-1 and NRI-2 struc- tures, p 1 D p 2 . Accordingly, these two parallel indexing structures are the simplest form of parallel index. The mechanism of index maintenance for these two parallel indexing structures is carried out as per normal index maintenance on sequential processors. The insertion and deletion procedures are summarized as follows. After a new record has been inserted to the appropriate processor, a new index key is inserted to the index tree also at the same processor. The index key insertion steps are as follows. First, search for an appropriate leaf node for the new key on the index tree. Then, insert the new key entry to this leaf node, if there is still space in this node. However, if the node is already full, this leaf node must be split into two leaf nodes. The first half of the entries are kept in the original leaf node, and the remaining entries are moved to a new leaf node. The last entry of the first of the two leaf nodes is copied to the nonleaf parent node. Furthermore, if the nonleaf parent node is also full, it has to be split again into two nonleaf nodes, similar to what occurred with the leaf nodes. The only difference is that the last entry of the first node is not copied to the parent node, but is moved. Finally, a data pointer is established from the new key on the leaf node to the record located at the same processor. The deletion process is similar to that for insertion. First, delete the record, and then delete the desired key from the leaf node in the index tree (the data pointer is to be deleted as well). When deleting the key from a leaf node, it is possible that the node will become underflow after the deletion. In this case, try to find a sibling leaf node (a leaf node directly to the left or to the right of the node with underflow) and redistribute the entries among the node and its sibling so that both are at least half full; otherwise, the node is merged with its siblings and the number of leaf nodes is reduced. Maintenance of the NRI-3 structure is more complex because p 1 6D p 2 .This means that the location of the record to be inserted/deleted may be different from the index node insertion/deletion. The complexity of this kind of index mainte- nance is that the data pointer crosses the processor boundary. So, after both the record and the index entry (key) have been inserted, the data pointer from the new index entry in p 1 has to be established to the record in p 2 . Similarly, in the dele- tion, after the record and the index entry have been deleted (and the index tree is restructured), the data pointer from p 1 to p 2 has to be deleted as well. Despite some degree of complexity, there is only one data pointer for each entry in the leaf nodes to the actual record. 7.3.2 Maintaining a Parallel Partially Replicated Index Following the first issue on p 1 D p 2 mentioned in the previous section, mainte- nance of PRI-1 and PRI-2 structures is similar to that of NRI-1 and NRI-2 where Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 7.3 Index Maintenance 183 p 1 D p 2 . Hence, there is no additional difficulty to data pointer maintenance. For PRI-3, it is also similar to NRI-3; that is, p 1 6D p 2 . In other words, data pointer maintenance of PRI-3 has the same complexity as that of NRI-3, where the data pointer may be crossing from one processor (index node) to another processor (record). The main difference between the PRI and NRI structures is very much related to the second issue on single/multiple processors being involved in index restructur- ing. Unlike the NRI structures, where only single processors are involved in index maintenance, the PRI structures require multiple processors to be involved. Hence, the complexity of index maintenance for the PRI structures is now moved to index restructuring, not so much on data pointers. To understand the complexity of index restructuring for the PRI structures, con- sider the insertion of entry 21 to the existing index (assume the PRI-1 structure is used). In this example, we show three stages of the index insertion process. The stages are (i) the initial index tree and the desired insertion of the new entry to the existing index tree, (ii) the splitting node mechanism, and (iii) the restructuring of the index tree. The initial index tree position is shown in Figure 7.14(a). When a new entry of 21 is inserted, the first leaf node becomes overflow. A split of the overflow leaf node is then carried out. The split action also causes the nonleaf parent node to be overflow, and subsequently, a further split must be performed to the parent node (see Fig. 7.14(b)). Not that when splitting the leaf node, the two split leaf nodes are replicated to processors 1 and 2, although the first leaf node after the split contains entries of the first processor only (18 and 21—the range of processor 1 is 1–30). This is because the original leaf node (18, 23, 37) has already been replicated to both processors 1 and 2. The two new leaf nodes have a node pointer linking them together. When splitting the nonleaf node (37, 48, 60) into two nonleaf nodes (21; 48, 60), processor 3 is involved because the root node is also replicated to processor 3. In the implementation, this can be tricky as processor 3 needs to be informed that it must participate in the splitting process. An algorithm is presented at the end of this section. The final step is the restructuring step. This step is necessary because we need to ensure that each node has been allocated to the correct processors. Figure 7.14(c) shows a restructuring process. In this restructuring, the processor allocation is updated. This is done by performing an in-order traversal of the tree, finding the range of the node (min, max), determining the correct processor(s), and reallocat- ing to the designated processor(s). When reallocating the nodes to processor(s), each processor will also update the node pointers, pointing to its local or neighbor- ing child nodes. Note that in the example, as a result of the restructuring, leaf node (18, 21) is now located in processor 1 only (instead of processors 1 and 2). Next, we present an example of a deletion process, which affects the index structure. In this example, we would like to delete entry 21, expecting to get the original tree structure shown previously before entry 21 is inserted. Figure 7.15 shows the current tree structure and the merge and collapse processes. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 184 Chapter 7 Parallel Indexing Processors 1, 2 o18 o23 o37 o65 o71 o92 o46 o48 37 48 60 (a) Initial Tree o56 o59 o60 Processor 2 Processor 2 Processor 3 Processors 1,2, 3 Insert 21 (overflow) (b2) Split (Non Leaf Node) Processors 1, 2 o23 o37 37 48 60 (b1) Split (Leaf Node) Processors 1, 2, 3 Insert 21 (overflow) o18 o21 Processors 1, 2 Processor 2 o23 o37 48 60 Processors 1, 2, 3 o18 o21 Processors 1, 2 Processor 2 21 37 o46 o48 o46 o48 (c) Restructure (Processor Re-Allocation) o23 o37 48 60 Processors 1, 2, 3 o18 o21 Processors 1, 2 Processor 2 21 37 Processors 1, 2 Processors 2, 3 Processor 1 o46 o48 Figure 7.14 Index entry insertion in the PRI structures As shown in Figure 7.15(a), after the deletion of entry 21, leaf node (18) becomes underflow. A merging with its sibling leaf node needs to be carried out. When merging two nodes, the processor(s) that own the new node are the union of all processors owning the two old nodes. In this case, since node (18) is located in processor 1 and node (23, 37) is in processors 1 and 2, the new merged node Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 7.3 Index Maintenance 185 (a) Initial Tree o23 o37 48 60 Processors 1, 2, 3 o18 o21 Processors 1, 2 o56 o59 o60 Processor 2 21 37 Processors 1, 2 Processors 2, 3 Processor 1 D elete 21 (underflow) o65 o71 o92 Processor 3 o46 o48 Processor 2 Processors 1, 2 (b) Merge 48 60 Processors 1, 2, 3 37 37 Processors 1, 2 Processors 2, 3 Modify o46 o48 Processor 2 o18 o23 o37 void Processors 1, 2 o18 o23 o37 o46 o48 37 48 60 (c) Collapse Processor 2 Processors 1, 2, 3 Figure 7.15 Index entry deletion in PRI structures (18, 23, 37) should be located in processors 1 and 2. Also, as a consequence of the merging, the immediate nonleaf parent node entry has to be modified in order to identify the maximum value of the leaf node, which is now 37, not 21. As shown in Figure 7.15(b), the right node pointer of the nonleaf parent node (37) becomes void. Because nonleaf node (37) has the same entry as its parent node (root node (37)), they have to be collapsed together, and consequently a new nonleaf node (37, 48, 60) is formed (see Fig. 7.15(c)). The restructuring process is the same as for the insertion process. In this example, however, processor allocation has been done correctly and hence, restructuring is not needed. Maintenance Algorithms As described above, maintenance of the PRI structures relates to splitting and merging nodes when performing an insertion or deletion operation and to restruc- turing and reallocating nodes after a split/merge has been done. The insertion and deletion of a key from an index tree are preceded by a searching of the node where Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 186 Chapter 7 Parallel Indexing the desired key is located. Algorithm find_node illustrates a key searching proce- dure on an index tree. The find_node algorithm is a recursive algorithm. It basi- cally starts from a root node and traces into the desired leaf node either at the local or neighboring processor by recursively calling the find_node algorithm and pass- ing a child tree to the same processor or following the trace to a different processor. Once the node has been found, an operation insert or delete can be performed. After an operation has been carried out to a designated leaf node, if the node is overflow (in the case of insertion) or underflow (in the case of deletion), a split or a merge operation must be done to the node. Splitting or merging nodes are performed in the same manner as splitting or merging nodes in single-processor systems (i.e., single-processor B C trees). The difficult part of the find_node algorithm is that when splitting/merging nonleaf nodes, sometimes more processors need to be involved in addition to those initially used. For example, in Figure 7.14(a) and (b), at first processors 1 and 2 are involved in inserting key 21 into the leaf nodes. Inserting entry 21 to the root node involves processor 3 as well, since the root node is also replicated to pro- cessor 3. The problem is how processor 3 is notified to perform such an operation while only processors 1 and 2 were involved in the beginning. This is solved by activating the find_node algorithm in each processor. Processor 1 will ultimately find the desired leaf node (18,23,37) in the local processor, and so will processor 2. Processor 3 however, will pass the operation to processor 2, as the desired leaf node (18,23,37) located in processor 2 is referenced by the root node in proces- sor 3. After the insertion operation (and the split operation) done to the leaf nodes (18,23,37) located at processors 1 and 2 has been completed, the program control is passed back to the root node. This is due to the nature of a recursive algorithm, where the initial copy of the algorithm is called back when the child copy of the process has been completed. Since all processors were activated in the beginning of the find node operation, each processor now can perform a split process (because of the overflow to the root node). In other words, there is no special process whereby an additional processor (in this case processor 3) needs to be invited or notified to be involved in the splitting of the root node. Everything is a consequence of the recursive nature of the algorithm which was initiated in each processor. Figure 7.16 lists the find_node algorithm. After the find_node algorithm (with an appropriate operation: insert or delete), it is sometimes necessary to restructure the index tree (as shown in Fig. 7.14(c)). The restructure algorithm (Fig. 7.17) is composed of three algorithms. The main restructure algorithm calls the inorder algorithm where the traversal is done. The inorder traversal is a modified version of the traditional inorder traversal, because an index tree is not a binary tree. For each visit to the node in the inorder algorithm, the proc_alloc algo- rithm is called, for the actual checking of whether the right processor has been allocated to each node. The checking in the proc_alloc algorithm basically checks whether or not the current node should be located at the current proces- sor. If not, the node is deleted (in the case of a leaf node). If it is a nonleaf node, a careful checking must be done, because even when the range of (min,max) is not Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 7.3 Index Maintenance 187 Algorithm: Find a node initiated in each processor find  node (tree, key, operation) 1. if (key is in the range of local node) 2. if (local node is leaf) 3. execute operation insert or delete on local node 4. if (node is overflow or underflow) 5. perform split or merge on leaf 6. else 7. locate child tree 8. perform find  node (child, key, operation) 9. if (node is overflow or underflow) 10. perform split or collapse on non-leaf 11. else 12. locate child tree in neighbour 13. perform find  node (neighbour, key, operation) Figure 7.16 Find a node algorithm Algorithm:Index restructuring algorithms restructure (tree) // Restructure in each local processor 1. perform inorder (tree) inorder (tree) // Inorder traversal for non-binary // trees (like B C trees) 1. if (local tree is not null) 2. for i D1 to number of node pointers 3. perform inorder (tree!node pointer i ) 4. perform proc  alloc (node) proc  alloc (node) // Processor allocation 1. if (node is leaf) 2. if ((min,max) is not within the range) 3. delete node 4. if (node is non-leaf) 5. if (all node pointers are either void or point to non local nodes) 6. delete node 7. if (a node pointer is void) 8. re-establish node pointer to a neighbor Figure 7.17 Index restructuring algorithms Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 188 Chapter 7 Parallel Indexing exactly within the range of the current processor, it is not necessary that the node should not be located in this processor, as its child nodes may have been correctly allocated to this processor. Only in the case where the current nonleaf node does not have child nodes should the nonleaf node be deleted; otherwise, a correct node pointer should be reestablished. 7.3.3 Maintaining a Parallel Fully Replicated Index As an index is fully replicated to all processors, the main difference between NRI and FRI structures is that in FRI structures, the number of data pointers coming from an index leaf node to the record is equivalent to the number of processors. This certainly increases the complexity of maintenance of data pointers. In regard to involving multiple processors in index maintenance, it is not as complicated as in the PRI structures, because in the FRI structures the index in each processor is totally isolated and is not coupled as in the PRI structures. As a result, any extra complication relating to index restructuring in the PRI structures does not exist here. In fact, index maintenance of the FRI structures is similar to that of the NRI structures, as all indexes are local to each processor. 7.3.4 Complexity Degree of Index Maintenance The order of the complexity of parallel index maintenance, from the simplest to the most complex, is as follows. ž The simplest forms are NRI-1 and NRI-2 structures, as p 1 D p 2 and only single processors are involved in index maintenance (insert/delete). ž The next complexity level is on data pointer maintenance, especially when index node location is different from based data location. The simpler one is the NRI-3 structure, where the data pointer from an index entry to the record is 1-to-1. The more complex one is the FRI structures, where the data pointers are N-to-1 (from N index nodes to 1 record). ž The highest complexity level is on index restructuring. This is applicable to all three PRI structures. 7.4 INDEX STORAGE ANALYSIS Even though disk technology and disk capacity are expanding, it is important to analyze space requirements of each parallel indexing structure. When examining index storage capacity, we cannot exclude record storage capacity. Therefore, it becomes important to include a discussion on the capacity of the base table, and to allow a comparative analysis between index and record storage requirement. In this section, the storage cost models for uniprocessors is first described. These models are very important, as they will be used as a foundation for indexing the storage model for parallel processors. The storage model for each of the three parallel indexing structures is described next. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 7.4 Index Storage Analysis 189 7.4.1 Storage Cost Models for Uniprocessors There are two storage cost models for uniprocessors: one for the record and the other for the index. Record Storage There are two important elements in calculating the space required to store records of a table. The first is the length of each record, and the second is the blocking fac- tor. Based on these two elements, we can calculate the number of blocks required to store all records. The length of each record is the sum of the length of all fields, plus one byte for deletion marker (Equation 7.1). The latter is used by the DBMS to mark records that have been logically deleted but have not been physically removed, so that a rollback operation can easily be performed by removing the deletion code of that record. Record length D Sum of all fields C 1 byte Deletion marker (7.1) The storage unit used by a disk is a block. A blocking factor indicates the max- imum number of records that can fit into a block (Equation 7.2). Blocking factor D floor.Block size=Record length/ (7.2) Given the number of records in each block (i.e., blocking factor), the number of blocks required to store all records can be calculated as follows. Total blocks for all records D ceiling.Number of records=Blocking factor/ (7.3) Index Storage There are two main parts of an index tree, namely leaf nodes and nonleaf nodes. Storage cost models for leaf nodes are as follows. First, we need to identify the number of entries in a leaf node. Then, the total number of blocks for all leaf nodes can be determined. Each leaf node consists of a number of indexed attributes (i.e., key), and each key in a leaf node has a data pointer pointing to the corresponding record. Each leaf node has also one node pointer pointing to the next leaf node. Each leaf node is normally stored in one disk block. Therefore, it is important to find out the number of keys (and their data pointers) that can fit into one disk block (or one leaf node). Equation 7.4 shows the relationship between number of keys in a leaf node and the size of each leaf node. . p leaf ð .Key size C Data pointer// C Node pointer Ä Block size (7.4) Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [...]... spread spread spread, but spread not random randomly randomly not random randomly randomly not random randomly Figure 7.26 A comparative table for parallel multi-index selection query processing using a one-index access method NRI-1 Isolated record loading 212 Chapter 7 Parallel Indexing Both of these factors determine the efficiency of parallel multi-index search query processing based on the one-index... indexing structure in the context of parallel search and join query processing 7.7.1 Comparative Analysis of Parallel Search Index In this section, parallel one-index and multi-index search query processing are examined, followed by some discussions Analyzing Parallel One-Index Search Query Processing As mentioned previously, in parallel one-index search query processing there are three main elements: (i/... parallel join processing without indexes (e.g., parallel hash join) is applied instead 7.7 COMPARATIVE ANALYSIS As there are different kinds of parallel indexing structures and consequently various parallel algorithms for search and join queries involving index, as studied in the previous sections, it becomes important to analyze the efficiency of each parallel indexing structure in the context of parallel. .. of parallel multi-index search query processing Therefore, further performance analysis, incorporating storage space analysis and other operation analysis, is necessary in order to identify the efficiency of these indexing structures 7.7.2 Comparative Analysis of Parallel Index Join In this section, parallel one-index and multi-index join query processing are examined In parallel one-index join processing, ... loaded, and there is a great chance the records need to be loaded remotely and this incurs overhead Comparing PRI-1 with PRI-3, both seek remote index searching and data loading in parallel one-index join Additionally, PRI-3 needs remote record loading in parallel two-index join FRI-1 and FRI-3 are similar, since both require remote data loading in parallel one-index join and searching for starting and. .. offer a great number of benefits in parallel processing, which in this case is confirmed in parallel one-index join and parallel two-index join processing An obvious drawback is storage overhead, which can be enormously large If storage overhead is to be minimized, the nonreplicated parallel indexing structures, in particular NRI-1 and NRI-3, are favorable On the other hand, the PRI structures do not seem... to maintain the index On the other hand, NRI-1 is sufficient to provide support for parallel one-index search query processing The second preferable indexing structure to support parallel search query processing cannot be easily identified NRI/PRI/FRI-3 indexing structures are quite favorable, particularly in parallel one-index search query processing On the other hand, NRI/PRI-2 indexing structures are... search attributes are indexed, parallel algorithms for these search queries are very much influenced by the indexing structures Depending on the number of attributes being searched for and whether these attributes are indexed, parallel processing of search queries using parallel index can be categorized into two types of searches, namely (i/ parallel one-index search and (ii) parallel multi-index search... table comparison, each parallel indexing structure has advantages and disadvantages in supporting parallel index-join processing There is no single parallel indexing structure that is the most efficient in all aspects However, it is noted that NRI-2 and PRI-2 do not support parallel index-join processing efficiently Therefore, the use of these parallel indexing structures is not suggested The comparison shows... indexed table in both parallel one-index and parallel two-index join processing The efficiency of remote data loading is very much determined by the selectivity factor of the query The higher 214 Figure 7.27 A comparative table for parallel index-join query processing Local join Data partitioning Indexed table searching Indexed table record loading Parallel Merging Searching start and end values Two-Index . across to another processor. 7.5 PARALLEL PROCESSING OF SEARCH QUERIES USING INDEX In this section, we consider the parallel processing of search queries involving index predicates on multiple indexed attributes. 7.5.1 Parallel One-Index Search Query Processing Parallel processing of a one-index selection query exists in